The invention comprises a process for producing organic chemicals by selective oxidation where the selective oxidation reaction is conducted using porous particles comprising specially synthesized nanometer-sized crystallites of supported noble metal catalyst. Classes of chemical substrates which can be selectively oxidized by the process of the invention include alkanes, olefins, alcohols, aromatics, ketones, aldehydes as well as compounds containing mixed functionality and/or heteroatoms such as sulfur or nitrogen.
Selective oxidation reactions are a major class of chemical transformations accounting for the production of a wide variety of important chemical products, including epoxides, hydroxylates, alcohols, carbonyl compounds, acids, glycols and glycol ethers, oximes, lactones, and oxygenated sulfur and nitrogen compounds such as sulfoxides, sulfones, nitrones, azo compounds, and other N-oxides. Normally, performing these transformations efficiently and economically requires that a catalyst be used which allows the reaction to occur at a sufficiently high rate (activity) and favors the formation of the desired products (selectivity). Frequently, these catalysts are based, at least in part, on the use of a noble metal constituent such as platinum, palladium, iridium, rhodium, ruthenium, gold, osmium, and the like. Noble metals tend to have favorable activity and selectivity toward desired oxidative reactions. The noble metal may be used as a soluble complex (homogeneous catalyst), but it is also frequently used as a heterogeneous catalyst with the noble metal deposited onto a porous support.
Commonly, selective oxidation reactions are performed utilizing oxygen as the oxidizing agent. However, producing purified oxygen is expensive, requiring large capital investment and operating costs. Also, processes using purified oxygen combined with organic chemical feedstocks may accidentally achieve gas compositions in the explosive range, thereby posing a serious safety hazard. In other cases, selective oxidation processes utilize air as the oxidizing agent. But a major economic problem associated with such processes utilizing air is handling the accompanying undesired large flow of nitrogen which substantially increases process costs. Such oxidation processes can also be prone to forming explosive gas mixtures. Oxidative processes using oxygen or air also tend to suffer from product selectivity problems related to overoxidation of the organic chemical feedstock, normally producing undesired carbon oxides (CO, CO2).
An attractive alternative to using oxygen or air as the oxidizing agent is the use of peroxidic compounds to provide the reactive oxygen needed for oxidative transformations. One common version is the use of organic hydroperoxides as oxidizing agents. These hydroperoxide compounds, typically generated by air- or O2-oxidation of suitable intermediates, are reacted with chemical feedstocks to form oxygenated products and organic by-products. However, these organic by-products represent a significant disadvantage for processes of this type because a large amount of organic material must be recovered, either for recycle or for sale as a secondary product. In some cases, the amount of this secondary product is greater than the amount of the primary oxygenated product, and is typically a less desirable product. For example, conventional production of propylene oxide also results in production of large amounts of styrene or tert-butyl alcohol co-products which typically must be marketed in economically unpredictable markets. Furthermore, processes involving organic hydroperoxide intermediates pose significant safety hazards. Generating hydroperoxides requires reactions of air with organic chemicals which may form explosive mixtures. Furthermore, organic peroxides can themselves be explosive, particularly if they are accidentally concentrated above a certain critical concentration level.
Instead of using organic peroxides, hydrogen peroxide is a known desirable oxidizing agent. The byproduct of oxidation reactions using hydrogen peroxide is typically water, a safe compound that can be easily recovered and reused or disposed. The amount of water on a weight basis is much less than the amount of organic by-product when organic hydroperoxides are used, and thereby represents significant savings in process costs. However, past attempts to develop selective chemical oxidation processes based on hydrogen peroxide have encountered significant difficulties. Conventional hydrogen peroxide production utilizes the anthraquinone process, wherein the anthraquinone is first hydrogenated to hydroanthraquinone and then autoxidized to release hydrogen peroxide and the anthraquinone for recycle. Hydrogen peroxide is generated at low concentrations in the solution, and very large flows of anthraquinone and anthrahydroquinone must be handled in order to produce the desired hydrogen peroxide product. Accordingly, such conventionally produced hydrogen peroxide is generally too expensive for commercial use as an oxidizing agent for selective chemical oxidation processes.
An important alternative to the use of organic peroxides for the oxidation of organic compounds is the generation of hydrogen peroxide directly by the noble metal catalyzed reaction of hydrogen and oxygen. This approach avoids the difficulty of the accompanying large flows of a working solution and can reduce the cost of hydrogen peroxide. The prior art includes a number of catalytic technologies which directly convert hydrogen and oxygen to hydrogen peroxide, but generally utilize a hydrogen/oxygen feed wherein the hydrogen concentration is greater than about 10 mol %. These hydrogen concentrations are well above the flammability limit of about 5 mol % for such mixtures and create a serious process hazard with added process costs and capital equipment costs required to mitigate the explosive hazard. At hydrogen feed concentrations below 5 mol %, the prior art catalysts are not sufficiently active and selective to generate hydrogen peroxide product at a reasonable rate.
Recently, an improved process for direct catalytic production of hydrogen peroxide utilizing an active supported phase-controlled noble metal catalyst has been disclosed in applicants"" U.S. Pat. No. 6,168,775 B1, incorporated herein by reference in its entirety. Employing the catalyst and process taught in the ""775 patent, the foregoing problems and limitations in the manufacture of hydrogen peroxide have been overcome. Advantageously, the ""775 catalyst is highly active and produces hydrogen peroxide from hydrogen and oxygen with superior selectivity over the prior art processes. Of special importance, the process of the ""775 patent converts hydrogen and oxygen to hydrogen peroxide with high selectivity wherein the process hydrogen concentration is well below its flammability limit.
Various oxidation processes for organic chemical feedstocks utilizing hydrogen peroxide are known. For example, U.S. Pat. No. 4,701,428 discloses hydroxylation of aromatic compounds and epoxidation of olefins such as propylene using a titanium silicalite catalyst. Also, U.S. Pat. Nos. 4,824,976; 4,937,216; 5,166,372; 5,214,168; and 5,912,367 all disclose epoxidation of various olefins including propylene using titanium silicalite catalyst. European Patent No. 978 316 A1 to Enichem describes a process for making propylene oxide, including a first step for direct synthesis of hydrogen peroxide using a Pd catalyst, and a second step for epoxidation of propylene to form propylene oxide using titanium silicalite (TS-1) catalyst. However, the best hydrogen peroxide product selectivity reported is only 86%, based on the amount of hydrogen converted. In the process second step, the best selectivity of propylene oxide formation is 97%, based on hydrogen peroxide conversion. Therefore, the best overall yield of propylene oxide that can be achieved is 83%, based on hydrogen feed. However, higher yields of oxidized organic products are much desired, particularly when considering the relatively high cost of the hydrogen feedstock which is required for the direct catalytic synthesis of hydrogen peroxide.
While noble metals are often the preferred catalysts for selective oxidation reactions, their use is hindered by several factors. For example, because noble metals are extremely active oxidation catalysts, they can often over-oxidize the substrate, forming unacceptably high levels of by-products such as carbon oxides (CO and CO2). Only certain noble metal active sites or crystal faces give acceptable noble metal selectivity towards the desired products, but conventional methods for fabricating noble metal catalysts will often expose a substantial fraction of other, undesired active sites. These undesired sites may catalyze the formation of undesired by-products.
For example, many oxidation processes involve inserting oxygen at the end of the oxidizable organic molecular structure or substrate or, as often described, at the alpha carbon of the substrates. The desirable oxidative active site of the noble metal catalyst will be those metal atoms exposed on the crystal face in 110 planes. The top layer of the face 110 has the noble metal atoms configured in line, thereby allowing the adsorbed oxidizable substrates to be oxidized only from each end. However, for the crystal face 100 and 111, the metal atoms are not just exposed in a linear position; there are many metal atoms adjacent to every side of the central active site. The 100 and 111 crystal faces allow the adsorbed oxidizable substrate to be in contact with the oxidative agent such as oxygen from all directions, not merely the alpha carbon direction. This results in oxidative cleavage of the substrate to carbon oxides. Thus, a highly selective catalyst should expose mainly the desirable face 110, whereby excessive oxidation of the organic substrate can be avoided.
U.S. patent ""775 teaches that the exposition of noble metal crystal faces can be successfully controlled by using soluble polymers that form dispersible organo-metallic complexes with the noble metals in a solution which impregnates a nanometer-sized catalyst substrate with the noble metal. To produce nanometer-sized particles, the absorbed metal particle size is controlled by the polymer molecular weight used to form the metallo-organic complex. Since the particle size is determined by the number of the metal atoms it contains, by varying the polymer type and molecular weight the metal atom number in each particle is controlled; thus, a metal particle size in the nanometer range is produced having a controlled exposition for the catalytic reaction of hydrogen and oxygen to produce hydroperoxide.
Another problem related to noble metal catalysts is the very high cost of the noble metal. Generally, one tries to use the noble metal sparingly, but even so the cost of the noble metal itself can be a major factor in the economics of oxidation processes. In order to maximize the activity of the noble metal, extremely small metal crystallites are preferred. However, fabricating extremely small crystallites is difficult, and it is also difficult to prevent such crystallites from agglomerating to form larger particles.
Another difficulty related to the cost of noble metals is attrition. Especially when reactions are conducted in liquid medium, attrition of active metal from the catalyst will occur. Attrition rates of 5-10% per year or more are common in commercial practice. At the very high cost of noble metals, this represents a major loss of value.
The object of the present invention is to provide a method to selectively oxidize organic compounds in a process which overcomes the aforementioned problems extant in the processes of the prior art for the oxidation of organic compounds by peroxides, especially hydrogen peroxide. In particular, the objectives of the present invention include the realization of substantially superior selectivities for the oxidation of organic compounds in a system where the oxidation can be carried out safely in situ, i.e., in a single step concurrent with the production of hydrogen peroxide or, optionally, sequentially by selectively producing hydrogen peroxide followed by the oxidation of the organic compound in a second step or vessel. Either method enjoys the advantage of highly selective hydrogen peroxide formation wherein the hydrogen reactant is present at a concentration safely below its flammability limit of, typically, 5 mole percent. However, the processes can also be carried out effectively at hydrogen concentrations above 5 mole percent.
In particular, a process is disclosed for the oxidation of organic chemical(s) by hydrogen peroxide oxidizing agent produced in situ, hereinafter referred to as the in situ mode of the invention. The process comprises introducing feedstreams comprising a solvent, hydrogen and oxygen reactants and at least one oxidizable organic chemical into a vessel under oxidizing conditions in contact with nanometer-size crystals of supported particles of noble metal catalyst contained in the vessel and having the face of the noble metal crystals include expositions predominantly of the 110 and/or 220 type of crystal planes. Optionally, the in situ mode of the invention may include a second catalyst comprising an organic chemical(s) oxidation catalyst introduced into the reaction mixture. The second catalyst is preselected based on its activity to selectivity catalyze the hydrogen peroxide/organic chemical(s) oxidation reaction in the in situ system of the invention. In either configuration of the in situ process, the hydrogen and oxygen are substantially converted to the hydrogen peroxide oxidizing agent and the organic chemical(s) is oxidized in situ in contact with the hydrogen peroxide product. The oxidized organic chemical(s) product, catalyst(s), solvent, unconverted organic chemical(s), hydrogen and oxygen are separated and recovered.
The two-stage process, or two-stage mode, of the invention for the selective oxidation of organic chemical(s) feedstocks utilizing directly produced hydrogen peroxide intermediate as oxidant comprises feeding hydrogen and oxygen-containing gas together with a solvent into a first catalytic reactor containing nanometer-size crystals of supported particles of noble metal catalyst under oxidizing conditions maintained at 0-100xc2x0 C. temperature and 300-3,000 psig pressure, whereby hydrogen peroxide intermediate is formed at hydrogen concentrations below the flammability limits of hydrogen. The catalyst contains nanometer-size crystallite particles of supported noble metal catalyst having the face of the noble metal crystals include expositions predominantly of the 110 and/or 220 type of crystal planes;
The organic chemical feedstock and solvent plus said hydrogen peroxide intermediate in an amount sufficient to comprise 1-30 wt. % are introduced into the reaction mixture in a second catalytic reactor containing a second catalyst, preferably comprising titanium silicalite, under oxidizing conditions of 0-150xc2x0 C. temperature and 15-1,500 psi pressure. The chemical feedstock is oxidized to provide an oxidized organic chemical product which is separated and recovered
As those skilled in the art will appreciate, the two-stage mode of the invention may be carried as batch or continuous operations.